Time for another dirty secret of beauty. All this week we've been revealing ways to help you look your best. This morning our Becky Worley, always looking good, checking out online beauty bargains and what you might be sacrificing for the savings. ? Reporter: Makeup is expensive so to find a deal many are going online buying brand name products through third party vendors but a growing trend, those brand names bought online may actually be knockoffs. Dermatologists say these products may contain potentially dangerous amounts of chemicals or heavy metals like lead that can harm the skin. All these products look real online but how do you know? I ordered a bunch of cosmetics online from third party vendors selling brand name products and compared them to the same items I bought at authorized retail stores. The first red flag, my purchase from this outlet site which says it sells authentic Mac makeup for a fraction of the cost. The bill came from beijing, China. Many of the items I purchased online looked a little off. This is supposed to be a matte shade. That looks awfully shiny. The color is definitely different. Look at this imperfection above the "R." Two stood out. This Mac foundation from the site. S this there's a sticker and the mirror is cracked and this benefit, they're real mascara I purchased on Ebay for $6 less than the retail cost. Different eyes, they're green. They're brown. See the printing over here is just imperfect so we sent the mascara and foundation online to nst labs to see if they were comparable to the ones in the stores. Guess what, they weren't. The mascara wasn't the same as the authentic. Although the lab says the differences were slight, most staggering, the lead found in the knockoff Mac foundation. The authentic product had 0.27 micrograms per gram. The knockoff we purchased, 260 micrograms per gram of lead. Almost a thousand times as much. This equates to 26 times the allowable level of lead in cosmetics produced in Canada. Currently there are no set levels for lead or other heavy metals for cosmetics in the U.S. Reporter: Dermatologists say the trace amount of lead in the authentic foundation isn't concerning. But the amount in the knockoff compact is. Lead can be absorbed through the skin. It's not quite as direct as ingesting it but depending on the content some knockoffs can potentially cause serious reactions. They can permanently scar you if you get a reaction on your skin that is severe enough. Reporter: The fda leads it up to cosmetic companies to test the safety. Knockoffs rchts required to be tested at all. If they're not regulated or sold by a retailer that's designated to sell these products then you really don't know what air you're getting. The Mac outlet site didn't return our repeated calls and the corporate offices from both benefit and Mac urged their customers to purchase from authorized retailers and Ebay and Amazon say they take this seriously and even restricted the seller of that mascara we bought and they say they will refund your purchase if it's not as described. Good. It's really hard to tell. Okay. You're going to put me to the test. Knockoff or real. Just by the packaging? I'd say this is real and this is the knockoff. Completely wrong. And that's the point. You cannot tell. You have to buy from an authorized retailer. Okay, how is that? Completely wrong? I love you, man. Oh, wow. You got to purchase from the store. We got to do it. Hard to get a bargain online and know you're getting the real thing. That's true. You're the real deal.

This transcript has been automatically generated and may not be 100% accurate.

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